Object: To examine the distribution of mast cells in human placental tissue and to analyse the interactions between mast cell mediators (histamine and serotonin (5-HT)) and contractile activity of the uterus during deli very. Design: 3.4 placentas were examined. Histamine and serotonin levels in placental samples were estimated using fluorymetric method. Distribution of mast cells were analysed immunohistochemicaly on the same material. Setting: Dept. of Pathophysiology, Medical Academy, Warsaw, Poland. (*) Laboratory of Developmental Neuropathology Centre of Experimental and Clinical Medcine, Polish Academy of Sciences, Warsaw. (**) Clinic of Obstetrics and Gynecology, Second Medical Faculty, Medical Acamedy, Warsaw. Subjects: 34 women, whose pregnancies ended in time and in preterm delivery, with normal, abnormal or without contractile activity of the uterus (elective cesarean section). Main outcome measurements: Comparative charts of histamine levels in placental samples in relation to term and course of the labour. Results: Concentration of histamine in placental cuts obtained from maternal surface of the placenta was significantly higher in the case of delivery without contractile activity of the uterus (elective cesarean section). Immunohistochemical identification of mast cells proved, that in re­gions located close to fetal surface of the placenta and in connective tissue foci, mast cells were most numerous. Conclusions: It is postulated that the simultaneous release of mast cell mediators (especially histamine) in placental tissue could be an important factor for evoking contractile activity of the human uterus, initiating labour. Further investigations should give answers on the points of these dependences.